Process for making a phase retarder film

ABSTRACT

A phase retarder film made of a thermoplastic resin having an in-plane retardation of 50 to 300 nm and a ratio of the in-plane retardation to the retardation normal to the plane of not less than 0.5 but not more than 1.8, a process for the production thereof and a laminate phase retarder film using said phase retarder film above.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phase retarder film.

2. Description of the Related Art

Phase retarder films have been used as optical compensators for STNliquid crystal display devices and in more recent years their variousapplications to optical compensators for other types of liquid crystaldisplay device than the STN type have also been studied. For example,there have recently been cases that liquid crystal display devices ofthe STN and TN types are required to have specific viewing anglecharacteristics depending upon their application. For this reason thereis a need for many kinds of phase retarder films having opticalcharacteristics for respective applications. However, the currentlydeveloped phase retarder films are restricted to two types, i.e., onehaving characteristics close to those of the uniaxially orientated filmsused for the aforementioned STN liquid crystal display device and theother having characteristics close to those of the perfectly biaxiallyorientated films comprising a layered inorganic compound layer, in whichthe in-plane refractive index is different from the refractive index inthe direction of thickness, as described in EP-A-0541308. There have notbeen developed such phase retarder film as having characteristicsintermediate between these two types of phase retarder films so far.

Proceedings of Eurodisplay '93, p. 149 reports a simulation where aphase retarder film having refractive indices, n_(x) =1.618, n_(y)=1.606, and n_(z) =1.493 and a thickness of 9.296 μm is effective asoptical compensator for the bend orientation type OCB mode liquidcrystal display device (π cell). Calculation based on the data for thephase retarder film indicates that the in-plane retardation is 112 nmand the ratio of the in-plane retardation to the retardation normal tothe plane is 0.101.

However, the phase retarder films which have heretofore been used forthe STN liquid crystal display device have a ratio of the in-planeretardation to the retardation normal to the plane of not less than 2.0due to the uniaxially orientated structure of the films. On the otherhand, the phase retarder films comprising a layered inorganic compoundlayer having an in-plane refractive index and a refractive index normalto the plane which are different from each other can not be producedunless the in-plane retardation is in the range of 0 to 50 nm asdescribed in EP-A-0541308. Therefore, any phase retarder film havingcharacteristics useful as, for example, optical compensator for theaforementioned π cell can not be obtained from those films which havebeen developed for current mass-production of phase retarder films oreven from their combination. For this reason, there is a need to developphase retarder films having optical characteristics different from thoseof the conventional ones. Especially the development of phase retarderfilms made of light weight thermoplastic resins and a process forproducing efficiently and advantageously in industry such phase retarderfilms.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a phase retarder filmhaving an in-plane retardation of 50 to 300 nm and a ratio of thein-plane retardation to the retardation normal to the plane of not lessthan 0.5, but not more than 1.8.

Another object of the present invention is to provide a process forproducing a phase retarder film having an in-plane retardation of 50 to300 nm and a ratio of the in-plane retardation to the retardation normalto the plane of not less than 0.5, but not more than 1.8 usingthermoplastic resins.

Still another object of the present invention is to provide a laminatephase retarder film having a ratio of the in-plane retardation to theretardation normal to the plane of 0.03 or more to less than 0.3comprising a first phase retarder film having an in-plane retardation of50 to 300 nm and a ratio of the in-plane retardation to the retardationnormal to the plane of not less than 0.5, but not more tan 1.8 and asecond phase retarder film comprising a layered inorganic compound layerlaminated on said first phase retarder film.

Still another object of the present invention is to provide a processfor producing such a laminate phase retarder film having a ratio of thein-plane retardation to the retardation normal to the plane of 0.03 ormore to less than 0.3 comprising said first phase retarder film and saidsecond phase retarder film comprising a layered inorganic compound layerlaminated on said first phase retarder film.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present inventors have made a research to overcome the difficultiesas described above and successfully developed a phase retarder filmhaving an in-plane retardation of 50 to 300 nm and a ratio of thein-plane retardation to the retardation normal to the plane of not lessthan 0.5, but not more than 1.8, which an embodiment of the presentinvention is based on. Moreover, they have successfully developed alaminate phase retarder film having a ratio of the in-plane retardationto the retardation normal to the plane of 0.03 or more to less than 0.3comprising a first phase retarder film having an in-plane retardation of50 to 300 run and a ratio of the in-plane retardation to the retardationnormal to the plane of not less than 0.5, but not more than 1.8 and asecond phase retarder film comprising a layered inorganic compound layerlaminated on said first phase retarder film, which another embodiment ofthe present invention is based on.

Moreover, the present invention relates to a phase retarder film made ofa thermoplastic resin having an in-plane retardation of 50 to 300 nm anda ratio of the in-plane retardation to the retardation normal to theplane of not less than 0.5, but not more than 1.8 and a process forproducing the same. Furthermore, the present invention relates to aprocess for producing such a laminate phase retarder film having a ratioof the in-plane retardation to the retardation normal to the plane of0.03 or more to less than 0.3 comprising said first phase retarder filmand said second phase retarder film comprising a layered inorganiccompound layer laminated on said first phase retarder film.

The thermoplastic resins to be preferably used include those excellentin transparency, for example, polycarbonates, cellulose diacetate,polyvinylalcohols, polysulfones, polyethersulfones, polyarylates, andthe like. Among them polycarbonates and polysulfones are more preferred.

The thermoplastic resin films to be used may ordinarily be produced by asolvent casting process. The thickness of the thermoplastic resin filmsranges, for example, from 50 to 300 μm, preferably 100 to 200 μm.

The phase retarder films of the present invention may be produced by,for example, the following process:

The thermoplastic resin films produced by a solvent casting process aretransferred into a tenter comprising a train of a preheating section, astretching section and a heat-treating section and subjectedsequentially to preheating, uniaxially stretching and heat-treatingtherein. The term "preheating" as used here means a treatment topreviously soften the thermoplastic resin films by heating so that thefilms can be satisfactorily stretched in the next stretching treatment.

The thermoplastic resin films transferred into the apparatus arepreheated under the conditions of Tg<Tp≦(Tg+100° C.), where Tgrepresents the glass transition temperature of the thermoplastic resinand Tp represents the preheating temperature, and the preferredpreheating time is usually in the range of 0.1 to one minute in order tosoften the resin films to a requisite extent as well as to suppressdeformation of the resin films more than the required softening.

Next, the preheated films are transferred into the stretching sectionwhere they are uniaxially stretched in the transverse direction normalto the film proceeding direction at a deformation rate of 150%/min. to1000%/min. at a stretching ratio of 2 to 3 times the original widthunder the conditions of Tg<Ts≦(Tg+100° C.), where Ts represents thestretching temperature, with no shrinkage being caused in the directionnormal to the stretching direction. The stretching temperature, thedeformation rate and the stretching ratio are in principle selected byroutine procedures depending upon the kind and thickness of thethermoplastic resin films to be used as raw films, and the in-planeretardation, the ratio of the in-plane retardation to the retardationnormal to the plane and the thickness of the requisite phase retarderfilm, and the like. For example, when polycarbonates are used,preferably the stretching temperature is in the range from 190° to 220°C., the deformation rate from 150 to 600%/min. and the stretching ratiofrom 2 to 3 times. Generally a reduction in stretching temperature, oran increase in deformation rate, or an increase in stretching ratiotends to result in an increased in-plane retardation, and an increase instretching ratio tends to increase the ratio of the in-plane retardationto the retardation normal to the plane.

Next, the uniaxially stretched films are transferred into theheat-treating section where they are subjected to a temperatureretaining step, so-called heat-treatment, i.e., they are kept at atemperature in the range of (Ts-50° C.)≦Ths≦Ts, where Ths represents theheat-treatment temperature, usually for 0.1 to one minute with the chuckwidth, i.e., the stretched width of the films achieved after stretchingbeing kept in order to fix the orientation of the films. At this time,the films may shrink in the range from 0 to 10% in the stretchingdirection if necessary. When the shrinking is desired, it can beachieved by, for example, narrowing the chuck width to a desiredshrinking proportion.

Although the process for preheating, uniaxially stretching andheat-treating may be performed by any other technique than the use ofthe tenter as described above, the tenter is preferably employed in theindustrial production.

The thus produced phase retarder films should have an in-planeretardation of 500 nm 300 nm and a ratio of the in-plane retardation tothe retardation normal to the plane of not less than 0.5, but not morethan 1.8 so that they have quite different optical characteristics fromthose of conventional phase retarder films. The phase retarder film maybe referred to simply as "phase retarder film A" in the specification.

The in-plane retardation and the ratio of the in-plane retardation tothe retardation normal to the plane can be set to desired levels byroutine procedures depending upon the end-use to which the phaseretarder films are to be put. The in-plane retardation is preferably inthe range from 80 nm to 200 nm, and the retardation normal to the planeis preferably in the range from 45 nm to 400 nm. The thickness of thephase retarder films is preferably in the range from 50 to 150 μm inview of handling properties.

The phase retarder films A can be used alone or in combination withother phase retarder films as optical compensators for various liquidcrystal display devices.

For example, the phase retarder films A may be used in combination withthe phase retarder films having an in-plane retardation of 0 to 50 nmand a retardation normal to the plane of 50 nm to 1000 nm and comprisinga layered inorganic compound layer having an in-plane refractive indexand a refractive index normal to the plane which are different from eachother as disclosed in EP-A-0541308. The phase retarder film comprising alayered inorganic compound layer may be referred to simply as "phaseretarder film B" in the specification.

The phase retarder films B may be produced by dispersing the layeredinorganic compound into a solvent or swelling it with a solvent, thencoating and drying as described in EP-A-0541308.

The layered inorganic compounds to be used include, for example, clayminerals. Examples of layered inorganic compounds include preferablysynthesized sodium tetrasilicate mica with low impurity and smectitefamily minerals such as montmorillonite, beidelite, nontronite,saponite, hectorite, sauconite, and synthesized compounds having acrystalline structure similar to those of the smectite family.

The solvents to be used for swelling or dispersing the layered inorganiccompounds may be optionally selected from, for example,dimethylformamide, dimethylsulfoxide, nitromethane, water, methanol, andethylene glycol.

In the production of the layer of the layered inorganic compounds, asdescribed in EP-A-0541308, a dispersion of the layered inorganiccompounds may preferably be mixed with an optically transparenthydrophilic resin in order to improve layer-formability and physicalproperties such as resistance to crack of the layers. The ratio byvolume of the layered inorganic compound to the optically transparentresin may be, for example, in the range from 0.1 to 10. As an example ofoptically transparent hydrophilic resin, mention may be made ofpolyvinylalcohol.

A layered inorganic compound layer may be formed on an opticallytransparent resin film to produce a phase retarder film B reinforcedwith the optically transparent resin film. Phase retarder film B alsoimplies this laminate film in the present invention. This laminate filmmay be referred to simply as "phase retarder film B1" in thespecification.

The techniques for laminating the phase retarder film A and the phaseretarder film B are not critical. For example, a process comprisingforming a layered inorganic compound layer directly on a phase retarderfilm A, or a process comprising adhering a phase retarder film B1, asdisclosed in EP-A-0541308, onto a phase retarder film A with an adhesiveor sticking agent can be employed. The number of phase retarder films tobe laminated, optical characteristics of each phase retarder film, i.e.,the in-plane retardation, the ratio of the in-plane retardation to theretardation normal to the plane and the like may be optionally selecteddepending upon the optical characteristics required by the finallaminate phase retarder films.

When two or more phase retarder films A are used in the laminate phaseretarder film, the lamination between one phase retarder film A andother phase retarder film A can be performed in the manner that the slowaxis of one phase retarder film A is parallel or normal to that of theother phase retarder film A depending upon the optical characteristicsrequired by the final laminate phase retarder film.

When at least one laminate phase retarder film B1 having an in-planeretardation is laminated onto at least one phase retarder film A, thelamination between phase retarder film B1 and phase retarder film A orbetween one phase retarder film B1 and other phase retarder film B1, canbe performed in the manner that the slow axis of one phase retarder filmis parallel or normal to that of other phase retarder film dependingupon the optical characteristics required by the final laminate phaseretarder film, whereby the finally required optical characteristics canbe achieved.

The laminate phase retarder film produced by laminating the phaseretarder film A on the phase retarder film B can be used as an opticalcompensator having a ratio of the in-plane retardation to theretardation normal to the plane of 0.03 or higher to less than 0.3.

The phase retarder film A has novel optical characteristics such as anin-plane retardation of 50 to 300 nm and a ratio of the in-planeretardation to the retardation normal to the plane of not less than 0.5,but not more than 1.8, and can be used alone or in combination withother phase retarder films as optical compensators for various schemesof liquid crystal display device. Moreover, the laminate phase retarderfilm produced by laminating at least one phase retarder film A on atleast one phase retarder film B can be used as an optical compensatorhaving a ratio of the in-plane retardation to the retardation normal tothe plane of 0.03 or higher to less than 0.3. Furthermore, the presentinvention allows quite advantageously in industry the efficient andsteady mass-production with high productivity of the phase retarderfilms as described above.

The present invention will be illustrated in detail with reference toExamples without being limited thereto.

The in-plane retardation was determined by means of a polarizingmicroscope equipped with Senarmont compensator with the film plane beingnormal to the optical system. The retardation normal to the plane wasdetermined according to the following formula:

Retardation normal to the plane=[(n_(x) +n_(y))/2-n_(z) ] x d wheren_(x) represents the maximum refractive index in the film plane, n_(y)represents a refractive index normal to the n_(z) in the film plane,n_(z) represents a refractive index normal to the film plane and drepresents a thickness of the film.

The laminate phase retarder film comprising at least one phase retarderfilm A and at least one phase retarder film B is regarded as amono-layer phase retarder film as a whole, and the in-plane retardationand the retardation normal to the plane of the film were determined inthe same manner as above.

EXAMPLE 1

A continuous film of polycarbonate (Tg: 148° C.) having a thickness of140 μm was prepared by solvent casting process. This film wastransferred into a tenter having a preheat-treating section of 3 m inlength, a stretching section of 6 m and a heat-treating section of 3 mand subjected sequentially to preheat-treatment at a temperature of 220°C. for 20seconds, transverse uniaxially stretching treatment in thedirection normal to the film proceeding direction at a deformation rateof 200%/min., at a stretching temperature of 207° C. and at a stretchingratio of 2.2 times the original width, and then heat-treatment at atemperature of 180° C. for 20 seconds with no shrinkage in thestretching direction.

The thus obtained phase retarder film had a thickness of 58 μm, anin-plane retardation of 101 nm, a retardation normal to the plane of 143nm and a ratio of the in-plane retardation to the retardation normal tothe plane of 0.706.

EXAMPLE 2

A continuous film of polycarbonate (Tg: 148° C.) having a thickness of140 μm was prepared by solvent casting process. This film wastransferred into the tenter identical to that used in Example 1, andsubjected sequentially to preheat-treatment at a temperature of 220° C.for 20 seconds, transverse uniaxially stretching treatment in thedirection normal to the film proceeding direction at a deformation rateof 233%/min., at a stretching temperature of 200° C. and at a stretchingratio of 2.4 times the original width, and then heat-treatment at atemperature of 180° C. for 20 seconds with a shrinkage in the stretchingdirection of 8.3%.

The thus obtained phase retarder film had a thickness of 60 μm, anin-plane retardation of 117 nm, a retardation normal to the plane of 159nm and a ratio of the in-plane retardation to the retardation normal tothe plane of 0.736.

EXAMPLE 3

A continuous film of polycarbonate (Tg: 148° C.) having a thickness of185 μm was prepared by solvent casting process. This film was introducedinto the tenter identical to that used in Example 1, and subjectedsequentially to preheat-treatment at a temperature of 220° C. for 10seconds, transverse uniaxially stretching treatment in the directionnormal to the film proceeding direction at a deformation rate of 467%/min., at a stretching temperature of 208° C. and at a stretching ratioof 2.4 times the original width, and then heat-treatment at atemperature of 200° C. for 10 seconds with no shrinkage in thestretching direction.

The thus obtained phase retarder film had a thickness of 70 μm, anin-plane retardation of 119 nm, a retardation normal to the plane of 143nm and a ratio of the in-plane retardation to the retardation normal tothe plane of 0.832.

EXAMPLE 4

On the surface of a triacetylcellulose film having a thickness of 80 μm(available from Fuji Photo Film Co., Ltd. under the tradename FUJITAC)which surface had been subjected to saponification, an aqueousdispersion obtained by mixing an aqueous 5% dispersion of synthetichectorite (available from Laporte Absorbents Co., under the tradenameLAPONITE XLS) and an aqueous 2.5% solution of polyvinylalcohol having asaponification of 98.5% and a degree of polymerization of 300 (availablefrom Kuraray Co., Ltd. under the tradename POVAL 103) in a ratio of 3:7by volume was coated to a film thickness of 23 μm on dryness to producea phase retarder film comprising a triacetylcellulose film having anlayered inorganic compound layer formed thereon (referred to as Film Xhereunder). The film X had an in-plane retardation of 8 nm, aretardation normal to the plane of 370 nm and a thickness of 103 μm.

Two films X were laminated with the slow axis of each film beingparallel to each other using an acrylic adhesive, to which a phaseretarder film prepared in the same manner as in Example 2 was laminatedusing an acrylic adhesive with its slow axis being perpendicular to theslow axis of the film x to produce a laminate phase retarder film. Thelaminate phase retarder film had a thickness of 316 μm, an in-planeretardation of 105 nm, a retardation normal to the plane of 1012 nm anda ratio of the in-plane retardation to the retardation normal to theplane of 0.104.

EXAMPLE 5

The same procedure as in Example 4 was repeated, except that the filmthickness on dryness was 16 μm, to produce a phase retarder filmcomprising a layered inorganic compound layer (referred to as film Yhereunder). The film y had an in-plane retardation of 10 nm, aretardation normal to the plane of 305 nm and a thickness of 96 μm.

Onto the film Y was adhered a phase retarder film prepared in the samemanner as in Example 2 using an acrylic adhesive with its slow axisbeing perpendicular to the slow axis of the film Y to produce a laminatephase retarder film. The laminate phase retarder film had a thickness of211 μm, an in-plane retardation of 110 nm, a retardation normal to theplane of 545 nm and a ratio of the in-plane retardation to theretardation normal to the plane of 0.202.

What is claimed is:
 1. A process for producing a phase retarder filmhaving an in-plane retardation of 50 to 300 nm and a ratio of thein-plane retardation to the retardation normal to the plane of not lessthan 0.5, but not more than 1.8 comprising the steps of:(1) preheating athermoplastic resin film at a temperature Tp in the range ofTg<Tp≦(Tg+100° C.), where Tg represents the glass transition temperatureof the thermoplastic resin and Tp represents the preheating temperature,(2) stretching uniaxially the film from step (1) at a deformation rateof 150%/min. to 1000%/min. at a stretching ratio of 2 to 3 times theoriginal length at a temperature Ts in the range of Tg<Ts≦(Tg+100° C.),where Ts represents the stretching temperature, with no shrinkage beingcaused in the direction normal to the stretching direction, and (3)heat-treating the film from step (2) at a temperature Ths in the rangeof (Ts-50° C.)≦Ths≦Ts, where Ths represents the heat-treatmenttemperature.
 2. The process according to claim 1, wherein saidheat-treating is performed with a shrinkage in the stretching directionbeing not more than 10%.
 3. The process according to claim 1, whereinsaid thermoplastic resin film is produced by solvent casting process. 4.The process according to claim 1, wherein said thermoplastic resin is apolycarbonate resin or a polysulfone resin.
 5. The process according toclaim 1, wherein said in-plane retardation is in he range from 80 to 200nm.